1. Field of the Invention
The present invention relates to a liquid phase deposition process for depositing silicon dioxide (SiO.sub.2), more particularly to a SiO.sub.2 deposition process using silicic acid instead of SiO.sub.2 powder to saturate hydrofluorosilicic acid.
2. Description of the Prior Art
Silicon dioxide is the most widely used insulator in semiconductor devices. This material can be grown by many techniques, including thermal oxidation, chemical vapor deposition (CVD), sputtering, e-gun evaporation, and liquid phase deposition (LPD). Among these, LPD is a low temperature process which can grow silicon dioxide at temperatures lower than 50.degree. C. LPD has such advantages as the fact that the formation of the film is possible at a low temperature, the equipment is simple and cheap, and the throughput is high. Moreover, the qualities of the films are relatively good, enabling the films to be used for producing electronic components.
For example, Yoshitomi et al. use LPD-grown oxide as the insulator of a metal-oxide-semiconductor (MOS) capacitor. They have found that the quality of the oxide can be improved after it is annealed at 400.degree. C. in O.sub.2 for 30 minutes (Proceeding of 1992 International Electron Devices and Material Symposium, p.22, Taipei, Taiwan, R.O.C., 1992). LPD-grown oxide has also been used in polycrystalline silicon thin-film transistors and is proven to be suitable for fabrication of silicon devices (Yeh et al., IEEE Electron Device Letters, 14(8), p.403, 1993)
The technique of depositing a SiO.sub.2 film by LPD was first disclosed by Nagayama et al. (J. Electrochem. Soc., 135(8), p.2013 (1988)). The purpose of depositing the SiO.sub.2 film on a glass substrate is to prevent the out-diffusion of alkali ions from the substrate, which may degrade the performance of the liquid crystal display (LCD) fabricated on it. According to Nagayama et al., hydrofluorosilicic acid (H.sub.2 SiF.sub.6) is first diluted with water to 2M. Then silica (silicon dioxide) powder is added into this solution to saturate the solution at 35.degree. C. in the following equilibrium state: EQU H.sub.2 SiF.sub.6 +2 H.sub.2 O.revreaction.SiO.sub.2 +6 HF (1)
The residual silica powder is then filtered out and the temperature of the solution is raised to the growth temperature. Boric acid (H.sub.3 BO.sub.3) is added to H.sub.2 SiF.sub.6 to consume the hydrofluoric acid (HF) and supersaturate the solution with SiO.sub.2 in accordance with the following reaction: EQU H.sub.3 BO.sub.3 +4 HF.revreaction.BF.sub.4.sup.- +H.sub.3 O.sup.+ +2 H.sub.2 O (2)
Thus the equilibrium in equation (1) proceeds to the right-hand side, and the amount of SiO.sub.2 is increased to a supersaturation level, followed by the deposition of a SiO.sub.2 film on the surface of a substrate.
The Nagayama method suffers from two drawbacks: a long saturation time (about 16 hours), and a low deposition rate, i.e., about 10-30 nm/hr with the addition of 2-3.times.10.sup.-2 M boric acid. Actually, the deposition rate of SiO.sub.2 depends on both the saturation level of the treatment solution and the concentration of boric acid. Use of a large quantity of boric acid may enhance the deposition rate significantly, but the deposited films may have poor quality. Raising the saturation level of the solution is another method to enhance the deposition rate, however, this requires a much longer time.
Goda et al. in their U.S. Pat. No. 5,073,408 utilize the principle that the equilibrium of the solution represented by the equation (1) proceeds to the right-hand side, when the temperature of the solution is increased. A saturating agent such as boric acid is not needed. Again, the method has the disadvantage of a very low deposition rate (not more than 6 nm/hr).
In conclusion, the conventional LPD method for depositing SiO.sub.2 suffers from the problems of long saturation time and low SiO.sub.2 deposition rate.